1. Field of the Invention
The present invention relates to means for the transmission and synchronization of multiple-independent signals and in particular to means for maintaining the separation of multiple independent signals transmitted over a single link.
2. Description of the Prior Art
A variety of links, including laser and microwave, may be employed to provide communication between two remote locations transferring data, voice or the like. Information transferred in such a manner is subject to periodic interruptions, the nature of which will depend upon the frequency and physical characteristics of the type of signal propagation employed. For example, infrared laser transmissions may be interrupted by the passing of an opaque object (such as a bird) through the line of sight of laser transmitter and receiver. Transmissions of multiple time division multiplexed conversations over such a link encounter the problem of commencing and maintaining the proper ordering of the information at the receiver so that the transmissions may remain private and secure rather than intermingled. The maintenance of privacy is complicated by the inherent loss of synchronization which takes place when any multiplexed transmission is interrupted as above described.
Systems have been devised to maintain signal ordering. A duplex two-signal modulation scheme for microwave is mentioned in a paper by Goldberg and Bath ("Multiplex Employing Pulse-Time and Pulse Frequency Modulation", Proc. IRE, January 1949, at p. 22). This system uses modulation of carrier phase for one of the channels, a technique unavailable for use in solid-state pulsed lasers such as the gallium arsenide laser. Another system is mentioned in an article by Rochelle ("Pulse-Frequency Modulation", IRE Trans. on Space, Electronics and Telemetry, June 1962, at p. 107). The described system multiplexes several channels onto a single carrier, each distinguishable from the other by representation in the form of a baseband pulse series utilizing different repetition frequencies. Again, this method of multi-channel modulation is not suited to a pulsed carrier link due to the large amount of energy wasted in the pulse series. A number of other concepts are similar to this system, such as the addition of a channel preamble or a header frame of identifiable width to the multiplexed information. These systems utilize energy and bandwidth without increasing the information content of the transmission.